Sintered resistance body, preferably for use as heating element



Aug. 30, 1966 E. FITZER ETAL SINTERED RESISTANCE BODY, PREFERABLY FORUSE AS HEATING ELEMENT Filed Nov. 6, 1962 Fig. 1

United States. Patent 29 Claims. (Cl. 29-491) Our invention relates tosintered electric resistance bodies, such as resistance heatingelements, of molybdenum silicade and is described herein with referenceto the accompanying drawings in which FIGS. 1, 2 and 3 show threedifferent resistance heating elements whose sintered silicide bodycomprises a high-temperature portion 1 for working temperatures above1300 C. joined with two end portions 3 :for lower temperatures whichcarry respective terminal caps 4 of contact metal.

The sintered silicide material of such resistance bodies may alsocontain some aluminum in an amount of 0.1 to 5.0% by weight so as toform a molybdenum aluminosilicide, and the silicide material may alsocontain minor additions of high-melting oxides, for example one or moreof zirconium oxide, aluminum oxide, beryllium oxide, silicon oxide. Theterm moylbdenum silicide as used herein is understood to include suchand similar variants of the sintered material.

It is known that sintered bodies of molybdenum silicide are resistant tooxidation, burn-off or scaling at high temperatures up to 1700 C. It isalso known that they are destroyed at low temperatures of less than 700C. due to occurrence of intercrystal-line oxidation. When employing amolybdenum silicide heating element, such as in (form of elongated rodsas exemplified by the drawing, the high-temperature portion is exposed,as a rule, to operating temperatures between 1300 and 1700 C., whereasthe cold ends of the other portions are subjected only to temperaturesbetween 200 and 700 C.

Due to the reactivity of the silicide materials relative to all knownmetallic and electrically good conducting substances, it is not feasibleto supply the electric current through a silicide freeoxidation-resistant material up to within the high-temperature portionof the molybdenum silicide element. This makes it necessary to have thesilicide material extend out of the high-temperature zone proper intozones of lower temperatures, for exam- .ple normal room temperature, orat least into a zone in which temperatures of a few 100 C. obtain. Onlyatthese relatively low temperatures can the silicide material becontactedvwith the conventional metallic conductors, for example withaluminum of which the terminal caps 4 according to the drawing mayconsist, without the danger of causing a reaction resulting in silicideformation.

Molybdenum silicide has the 'i iiroperjty of protecting itself fromfurther oxidation or sealingin an oxidizing atmosphere at temperaturesabove 1300} C. by forming on its surface a coating of silicon dioxideglass which constitutes a firmly anchored surfia-ce glaze. Thisprotective sheath of glaze is self-healing. That is, it renews itself inuse above 1300 C. in the event it has become damaged or destroyed, forexample by mechanical attack, alternating temperature stresses orchemical reaction.

3,269,806 Patented August 30, 1966 However, such a selfhealingprotective sheath does not form itself on the portion of the heatingelement that during its operation remains at temperatures below 1300 C.It has become known, therefore, to subject such element portions, priorto using the element, to an annealing treatment at high temperatures inan oxidizing atmosphere in order to form the above-mentioned protectiveglaze coating on these portions also. However, there remains thedifliculty that such coating is not self-healing so that if the coatingin the end portions of the silicide body is mechanically damaged, theseend portions may rapidly become subjected to further damage anddestruction due to the low operating temperature. It has further beenfound that during operation of a silicide element for a few hundredhours, there occurs in the lowttemperature portion a crystallization ofthe roentgen-amorphous glaze sheath. That is, the glazed end portionsbecome gradually de-glazed. Such de-glazing is accelerated by contact ofthe siO -glaze with insulating parts of ceramic and by reaction of thesilicon oxide with the :glaze coating or with basic oxides. Suchdc-glazing is accompanied by iormat-ion of fissures, cracks and newpoints of attack for oxygen upon the underlying body of molybdenumsilicide. Particularly endangered are the contacting locations of thesilicide material with the metallic contact material as exemplified bythe contact locations between the end portions 3 and the terminal caps 4of the heating elements illustrated on the drawing.

For securing a good electric contact, the previously produced oxidiccoating must be removed at the contacting locality. Very often, however,such removal results in a fissures and cracks that permit ingress ofoxygen. This takes place particularly at the merger location between thesilicide-free contact material and the glazecoated end portion of theheater element because the metallic contact material is not bonded withthe ceramic coating. The contacting locations, therefore, are onlymechanically enveloped and thus are not protected from penetration ofoxygen. The oxygen of the ambient air can thus enter between the contactmaterial and the silicide end portions from which the coating isremoved, and this oxygen results in a progressing destruction of thecontacted ends ultimately leading to the interruption of the currentpassage between contact material and silicide body.

It has been proposed to prevent such low-temperature oxidation bydepositing a protective layer of silicon. This has the disadvantage of ahigh electrical resistance in comparison with the metallicallyconducting silicides, The impregnating treatment required for producingsuch a silicon coating is also difficult to perform in practice and inmany cases has increased the rather limited sensitivity to mechanicalshock of the molybdenum silicide sinter bodies.

If an attempt is made to employ molybdenumafree, silicon-containingmetallic materials for the abovernentioned end portions of theresistance bodies, then a sintered or welded junction between thesematerials and the silicide of the high-temperature portion becomeslocated in a working-temperature range above 700 C. However, at 1000 to1100 C. such metallic materials fusion-bonded with the molybdenumsilicide tend to enter into conversion reactions which may beaccompanied by considerable changes in structure that cause mechanical 3tensions in the body and thus render it more susceptible to fissurefonmation. The use of molybdenum silicidetfree materials for the endportions of resistance heating elements is therefore limited to specialtypes of furnaces Where the just mentioned temperature conditions arereliably excluded.

It is an object of our invention to devise a molydenum silicide sinteredresistance body, preferably for use as a heating element, thateliminates all of the above-mentioned deficiencies and difliculties ofthose heretofore known and is reliably applicable for operatingtemperatures above 1300 C. of its high-temperature portion without thedanger of causing damage to the end portions of the body regardless ofthe temperature conditions to which they may be subjected in aparticular use.

According to our invention, those portions of the molybdenum silicideresistance body (sintered) whose normal operating temperature does notexceed 1300 C., preferably those portions that are subjected tooperating temperatures below 1000 C., are given an additional oxideconstituent in an amount of 0.1 to 20%, preferably 3 to 12%, this oxideaddition having a melting point below l400 C. and being solidified fromthe molten condition so as to coat the silicide particles in the poresand at the surface of the sintered molybdenum silicide body.

The percentages just stated, as Well as all others mentioned in thisspecification, are by weight.

By virtue of the invention, the molybdenum silicide sinter body has itsend portion and its contacted terminal ends, which become subjected tolow operating temperatures, coated on the inner and outer surfaces bythe oxide addition with the result that a low-temperature oxidation isprevented without requiring the provision of a glaze coating produced athigh temperature. The above-mentioned formation of an SiO coating byoxidizing annealing of the particular molybdenum silicide portion forprotection from low-temperature oxidation thus is no longer necessary,although such a coating, if desired, may also be applied in conjunctionwith the present invention. For example, such additional SiO coating isof advantage in cases where it is necessary to envelop thelow-temperature molybdenum silicide portion of the body in ceramicmaterial. In such cases, the formation of an SiO coating has been foundfavorable for excluding any reaction of furnace masonry with thematerial of the heating element. However, if the SiO coating becomesde-glazed or fissured, the permanence of the underlying body accordingto the present invention remains preserved.

The trouble heretofore encountered at the contacting location of thesilicide bodies with the metallic conductors, for example between thesilicide body and its terminals of aluminum, is likewise at once fullyeliminated by virtue of the invention. We have found it to beparticularly advantageous if the additional oxide constituent of themolybdenum silicide material has a linear coefiicient of thermalexpansion from 5-10 to 8- 10* per degree C. within the temperature rangeof 20 to 1400 C. It has been ascertained that with such oxide additions,no mechanical destruction of the silicide body is encountered even withrepeated alternating temperature stresses regardless of an abruptapplication and change of such stresses.

The objective of the invention is achieved with particular reliabilityif the additional oxide constituent in the portion of the sinter bodyfor working temperatures below 1300 C. is distributed over the entiremass of this portion. Preferably the additional oxide constituent isquantitatively distributed uniformly over each cross section so as tosecure constant area density of the constituent. However, we have alsofound it to be preferable to distribute the quantity of the additionaloxide constituent within the just-mentioned portion of the silicide bodyso that the constituent exhibits a concentration gradient, namely sothat the oxide proportion increases with decreasing operatingtemperature. That is, the end portions of the sintered resistance bodypreferably have a r C. as required by our invention.

Percent A1 0 27.4 SiO 64.6 Li O 8 A1 0 25 SiO 67 K 0 3 CHFZ 5 It hasbeen discovered, surprisingly, that the molybdenum silicide sinterbodies containing oxide having a melting point below 1400 C. are veryinsensitive to impurities. In the course of the investigations, we havefound that many metallic impurities, for example zirconium, on the onehand, and non-metallic impurities, for example carbon or nitrogen, onthe other hand, are extremely troublesome in the natural protectiveaction mechanism of molybdenum silicide under SiO glaze formation.Minute quantities of impurities suffice to cause the formation ofvoluminous reaction products in the interior that causes bursting of thesinter body. Such destruction also occurs if high-melting oxides areadded to the molybdenum silicide sinter body. In no case, however, couldit be ascertained that such bursting, due to impurities in the sinterbody, took place in cases where this body was given an addition of oxidemelt-ing below 1400 A reliable explanation of this phenomenon is not yetavailable. It is surmised that the oxide melting below 1400 C. iscapable of dissolving from the impurities the reaction products thatcause the bursting effect and to convert them into a softening,plastically deformable oxide phase which prevents the explosive actionof such crystalline derivatives of the impurities.

It seems that in this respect the low softening point of the oxide phaseis essential, which oxide phase apparently absorbs the derivativeproducts already in the temperature range commencing at about 1000 C.,and hence at a time at which the rate of oxidation-product formationfrom the impurities is so great as to be capable of resulting in theformation of dangerous crystalline oxidation products.

In principle, such oxides, melting below 1400 C. and applicable for thepurposes of the invention, can be given any desired composition. It isonly required that they have acidic character, i.e. that acid-formingoxides are preponderant in the molar ratio.

Favorably employed, for example, has been the following oxide substancewhich melts at about 1380 C. and commences to soften already atconsiderably lower temperatures:

Percent A1 0 14.1 Si0 81.8 Li O 4.1

Further compositions will be presented hereinafter particularly withreference to Examples Nos. 1 to 5. (As mentioned, all percentages are byweight.)

Prior to melting, the meltable oxides pass through a softening phase.For the operation of a heating element according to this invention, itis favorable to have the operating temperature approach this softeningphase or to remain within the range of the softening phase because then,on the one hand, the chemical protection (prevention of dissociation atlow temperature) is better and, on the other hand, the end portion orconnecting end of the heating element becomes elastic and thus loses orreduces its sensitivity to impact. This is particularly important withrespect to alternating temperature stresses as applied to heatingelements which in operation are subjected to frequent electricenergizations and deenergization. On the other hand, it has been foundthat oxides whose melting point is too low may cause smearing or gluingof the enveloping ceramic and may also ooze out in furnace zones thatare to hot. Best applicable as oxides for the end portions of thesilicide bodies have been found those that have melting temperatures of1250 to 1350 C. An example of such an oxide substance, melting at about1280 C., is the following:

In heating elements according to the invention, the silicide material inthe high-temperature portion, as exemplified by the portions 1 on theaccompanying drawing, is preferably made of the ternary systemmolybdenum-silicon-aluminum. It is known that by partially substitutingsilicon by aluminum in molybdenum silicide, the high-temperatureresistance and the specific electric resistance of high-temperatureportion are increased. For that reason, such ternary silicidecompositions are preferred for heating conductors to be employed athighest temperatures, but the material may also contain oxidic ornon-metallic additions. On the other hand, it has been found that suchmolybdenum-silicon-aluminum compounds are particularly susceptible tolow-temperature dissociation under the effect of oxygen.

For that reason it is particularly preferable to employ for the purposesof our invention a combination of a ternary MoSi-Al high-temperatureportion with end portions that consist of molybdenum silicide andcontain oxides melting below 1400 C. It has also been found that oxidesmelting below 1400 C., if added to Mo-Si-Al alloys, also successfullyprevent low-temperature dissociation by oxidation of the ternarycompound. Consequently a resistance body according to the invention mayconsist in its high-temperature portion of an M0- Si-Al or Mo-siliciclealloy with or without high-melting oxides (melting point above 1400 C.)and whose lowtemperature or end portion contains an oxygen additionwhich melts below 1400 C.

The end portions of heating elements according to the invention can beproduced in various ways. For example, the pulverulent mixture of themolybdenum silicide base mass with the additional oxide constituent canin molten condition. This method has been used to advantage in caseswhere very low melting oxide mixtures employed for impregnation, forexample oxlidic substances that contain boric acid. However, whenimpregnating an oxidic sludge into the pores of a pre-sintered silicidebody, it is necessary to subject the impregnated body to annealingtreatment for melting the sludge.

Generally, however, it is recommended to mix intimately the molybdenumsilicide powder and the powder and the powder of the additional oxideconstituents, to shape the powder mixture, and to sinter the shapedbody. In this manner the desired homogeneous distribution of theadditional oxide substance over the entire cross section is mostreliably secured. It has been found, however, that oxidic additions donot always promote the sintering of molybdenum silicide. It is knownthat oxides available in form of a sol or gel, for example hydratizedsilicic acid, may promote the sintering of silicides. The pulverulentoxide components, however, may constitute an impendiment duringsintering operation. We have discovered that such impairment of thesintering ability can be obviated by slight additions of iron-groupmetals (iron, cobalt, nickel) to the silicide material. Well applicablein this respect, for example, have been additions of 0.01 to 1% of suchiron-group metals. Nickel has been discovered to be particularlyfavorable for promotion or acceleration of the sintering operation. Itmight be expected that such iron-group metal additions reduce theoxidation resistance of molybdenum silicide. With respect to themolybdenum silicide-oxide composition of resistance bodies according tothe invention, it has been unexpectedly determined that the iron-groupaddition has the opposite effect, namely of increasing the resistance ofoxidation. This is probably due to the accelerating effect upon thesintering process which obviates any inherent promotion of oxidation.Protection from oxidation is furthermore secured by the additional oxideconstituent even without the formation of the above-mentioned SiOprotective coating.

By virtue of the just-mentioned acceleration effect upon the sinteringof the silicide, we have succeeded, despite the additional oxidecons-titutent, in attaining in the end portions of the resistance body aspecific resistance smaller on the average than 0.4 ohm-mm. /m.Presented in the following are examples which will elucidate this bycomparing resistance bodies with and without additional oxideconstituent, and with and without addition of nickel.

It will be seen from the following table that the combination of theadditional oxide constituent and the mentioned metal constituentaccording to the invention has the further effect that the desiredresults to a large extent become independent of the choice of the grainsizes and their distribution.

be jointly shaped as a plastic mass by the extrusion method, and theextruded body can then be sintered. Good products have been made in thismanner, as will be described more in detail with reference to thefollowing Examples Nos. 1 to 5. Another way of production is toimpregnate a porous molybdenum silicide sinter body with the additionaloxide. This can be done by producing a sludge from the oxide in areadily evaporat- The specified substances in pulverulent form were g, gW g liq employing the impregnation mixed intensively for four hours indry condition. Thereafter 9% by weight of water, relative to the totalweight of the powder mixture, was added thereto and the mass wasmasticated until it was completely plastic. The mass was then extrudedto rod shape in the conventional manner with the aid of a piston press.The moist and readily deformable strands were first dried at roomtemperature for 24 hours and there-after heated in a drying cabinetwithin 10 hours from 20 to 105 C. and were thereafter kept for 12 hoursat 105 C.

Such pre-treated green bodies already had a relatively high strength.They were now sintered. Used for this purpose was a sintering furnaceheated with molybdenum wire. The sintering was performed for 1 hour at1350 C. under hydrogen. Thereafter the sintered rods were annealed at1350 C. for 1 hour in an oxidizing atmosphere (air). Thereafter the rodswere very densely sintered and coated by a glaze. The coating wasextremely resistant to changes in temperature and free of fissures. Therods had a volumetric weight of 5.2 g./cm. a bending strength of 1800kg/cm. and a specific resistance of 0.39 ohm-mm. /m.

Example 2 Constituents: Percent Molybdenum aluminum silicide 35,11.composed of 63% Mo, 36% Si, 1% Al 92.5 Kaolin 5.0 Feldspar 2.0 Fluorspar0.5

The pulverulent constituents were homogenized in dry condition for 24hours. Thereafter a aqueous polyvinyl alcohol solution was added inindividual portions and up to a total weight of 12% of the powdermixture. The mixture was oalendered until it was completely moistthroughout and ready to be extruded. The mass was extruded into rods inthe conventional manner and these were dried as described in Example 1.After drying, the bodies were sintered in hydrogen for 30 minutes at1300 C.

The rods thus produced were already found to glaze at 1100 C. in air andwere oxidation-resistant at operating temperatures up to 1200 C. Theirvolumetric weight was 5.2 g./cm. their bending strength 1800 kg./cm. andtheir specific resistance 0.4 ohm-mmP/m.

Example 3 Percent MoSi 40p 94 SiO -Ca 2 SiO 1 A1 O -gel 3 Thepulveru-lent constituents were processed in the same manner as describedin Example 1. The Al O -gel served as binding and plastifying agent. Thesintered bodies were found to have a volumetric weight of 5.5 g./cm. abending strength of 2400 kg./cm. and a specific resistance of 0.31ohm-mmP/m.

Example 4 Percent Mo-silicide powder, 40 91.7 Fe-powder 0.3 Mica, 4Quartz, 10,u. 4

The powder was first homogenized in dry condition. Polyvinyl alcohol andwater were added in small portions, and the mixture was plastified in amasticating and mixing machine. Thereafter the doughy mass was extrudedby means of a vacuum extrusion press. The extruded rods were dried inmagazines for 24 hours in air at room temperature and thereafter at 24hours at 125 C. The rods were then presintered in reducing atmosphere at1400 C. for 6 hours. Thereafter the rods were sintered in oxidizingatmosphere (air) for 2 hours at 1350 C.

The volumetric weight was found to be 5.6 g./cm.

the bending strength 2500 kg./cm. and the specific resistance 0.28ohm-mm. /m.

Mixture of 47.5% kaolin+47.5% feldspar+5% fluorspar 5.0

The dry mixture was plastified with aqueous alginate solution. Pressing,drying and reducing sintering was effected in accordance with Example 4.In some cases an oxidizing annealing was effected by thereafter heatingthe rods with the aid of electric current passing directly through therods.

The volumetric weight was found to be approximately 5.7 g./crn. thebending strength 2800 to 3000 kg./cm. and the specific resistance wasapproximately 0.26 ohmmm. /m.

Since the additional oxide constituents are intended only for the endportion of the silicide sinter body subjected to operating temperaturesbelow 1300 C., it is necessary to provide for a suitable junction of theend portion or portions with the high-temperature portion consisting ofmolybdenum silicide and, as the case may be, also of higher meltingoxide additions. Such as junction can be made by sintering or fusingeach end portion together with the high-temperature portion. Preferably,however, we provide between the end portion and the high-temperatureportion a disc of pressed molybdenum silicide powder which is insertedprior to the sintering or fusing operation and which, after thisoperation is performed, joins the two portions together by a fusion bondso that they form a seal integral structure. We have found that thesejunction discs can be given the same or approximately the same diameteras the end portion and the high-temperature portion and that thethickness of the discs is preferably made approximately equal to A to 1of their diameter.

It is particularly advantageous to dimension the junction discs so thattheir diameter, under consideration of their radial expansion during thejoining operation, is somewhat larger than the diameter of the adjacentbody portion so that the individual disc at the junction locationresults in a bulge relative to the body portions and thus envelops theadjacent ends of these portions. If desired, the bulge of the junctiondisc can subsequently be ground away.

It has further been ascertained as preferable to give the silicidematerial of the junction discs a similar oxide addition as theconnecting portions of the resistance body. Described in the followingare two examples involving junctions made by the disc method justmentioned.

Example 6 Two MoSi rods of circular cross section were used. One of themcontained the additional amount of oxide melting below 1400 C. to serveas end portion, whereas the other was to serve as high-temperatureportion. Both rods had a diameter of 5.5 mm. and were ground to planarshape at the ends to be joined with each other. These ends were clampedinto a welding apparatus so that the two planar surfaces were directlyin contact with each other. The two butt surfaces were then slightlymoved away from each other and a disc or tablet of 6 .mm. diameter and 1mm. height was inserted. Thereafter the rods were pressed toward eachother and against the tablet in air and without the use of anyprotective gas. Thereafter, electric current was passed longitudinallythrough the rods so that they became heated by the passage of currentuntil the welding location attained a temperature of 1750 C. Thereafterthe pressure against the rods was reduced to zero and the weldingtransformer of the apparatus gradually switched to lower current values.The butt welding operation per junction was perwide.

formed within 5 to 15 minutes depending upon the crosssectional area.

The welding tablet was produced as follows:

Prepared was a powder mixture of Percent MoSi in a grain size 40/.L 95.0Si +4.5 A1 0 +0.5

The powder mixture was moistened with a 5% polyvinyl alcohol solutionand then pressed in a die to the shape of tablets having theabove-mentioned dimensions. Thereafter the tablets were dried for 4hours at 50 C. and were then ready for use.

The butt-welded rods had a bending strength of 1600 to 1700 kg./cm. anda specific resistance of 0.24 to 0.25 ohm-mm. /m.

Example 7 Used were two rods of round cross section having a diameter of12 mm. One rod consisted of MoSi to serve as high-temperature portion.The other rod consisted of MoSi with an addition of oxide having amelting point below 1400 C. to serve as end portion. The butt faces ofthe rods were ground to planar shape and then clamped into a weldingapparatus vertically one above the other. Prior to pressing the rodsagainst each other, a tablet as described above having a diameter of 12mm. and a height of 1.5 mm. was inserted in unsintered condition. Thewelding location was then heated by highfrequency current up to 1600" C.without protective gas. Thereafter the temperature was reduced to normalroom temperature (20 C) within a few minutes. The junction thus producedwas found to be extremely fast at high as well as at low temperatures.The bending strength at room temperature was about 1800 to 2000 kg./cm.and the specific resistance was 0.24 to 0.26 ohm-mm. /m.

For welding oxide-free together with oxide-containing MoSi rods a tabletcomposition has been found to be well applicable as described above withreference to the production of the oxide-containing end portions inExamples 1 to 5. The production of the tablets is as described inExample 6.

In the following examples reference will be made to the three structuralembodiments of heating elements on molybdenum silicide base, asillustrated in FIGS. 1, 2 and 3 of the accompanying drawing.

Example 8 (FIG. I)

The high-temperature portion 1, according to FIG. 1,

constituting the heating element proper, consists of 90% MoSi and 10% ofa mixture composed of 75% SiO and 25% A1 0 This portion has a tubularcross section with an outer diameter of 6 mm. and an interior diameterof 3 mm. The tubular structure is provided with slits extendinglongitudinally and being spaced 50 mm. from each other, each slit being2 mm. long and 1 mm. Each connecting or end portion 3 of the heatingelement, subjected to operating temperatures below 1300 C. when inoperation, consists of approximately 94.75% MoSi +0.25% Ni+0.4% LiO+1.37% A1 O +3.23% SiO The end portions 3 have a cross section 'of12mm. diameter and a length of 150 mm. The portions 1 and 3 are joinedwith each other by a sintered junction 2 analyzed as follows:

95% MoSi +l% Al O +3.9% SiO +0.1% Fe The terminal caps 4 consist of purealuminum.

Example 9 (FIG. 2)

The M-shaped high-temperature portion 1 to operate at incandescenttemperatures has a full cross section of 5 mm. diameter. Its compositionis approximately 63% Mo, 35% Si and 2% Al. The shape of thehigh-temperature portion 1 is such that the distance between the twoparallel end portions 3 is smaller than the width of the heating elementin its high-temperature portion 1. Sintercd upon the ends of thehigh-temperature portion 1 are sleeves 5 of pure MoSi The end portions 3operating at temperatures below 1300 C. are composed as follows:

Percent MoSi 94 Fe 0.3 SiO 4 A1 0 1 CaO 0.7

The junction between high temperature portion 1 with sleeves 5 and theend portions 3 is made by a sintered composition of:

Percent MoSi 95 Si0 3.5 A1 0 0.5 CaO 0.5 Co 0.5

Example 10 (FIG. 3)

The high-temperature portion according to FIG. 3 comprises threeparallel tubes having an outer diameter of 10 mm. and an inner diameterof 5 mm. The tubes consist The upper ends of the tubular parts areconnected by a bridge piece 6 of pure sintered MoSi The bridge 6 hasrespective openings at the junctions with the tubes 1 so that theinterior of the tubes is in communication with the interior of thefurnace in which the heating element is to be used. This preventsbur-sting of the components by the formation of silicon monoxide insideof the tubes. The junction of the bridge 6 with the tubular parts andalso the junction between the tubular parts and the end portions 3, thelatter having a diameter of 18 mm., is made by a sinter junction of pureMoSi The length of the end portions 3 depends upon the dimensions of thefurnace insulation and, as a rule, is about 200 to 550 mm. The endportions consist of:

Percent MoSi 92 SiO 5 A1 0 1 CaO 1.50 BaO 0.25 Ni 0.25

It will be understood, of course, that our invention is not limited toany particular shape or dimensions of the heating elements or otherresistance bodies.

We claim:

1. A sintered electric resistance body having a hightemperature portionsuitable for operating temperatures above 1300 C. and end portions fortemperatures below 1300 C., said body consisting essentially ofmolybdenum silicide and containing in said entire end portions 0.1 to20% by weight of additional oxide substance that is a mixture of oxidesselected from the group consisting of oxides of Li, Na, K, Be, Ca, Ba,Al, B and Si and having a melting point below 1400 C., said oxidesubstance -being in the form of a coating on the silicide particles inthe pores and at the surface of the sintered body.

2. A sintered electric resistance body having a hightemperature portionsuitable for operating temperatures above 1300 C. and end portions fortemperatures below 1300 C., said body consisting essentially ofmolybdenum silicide with an addition of at least one high-melting oxide,and containing through-out said end portions 3 to 12% by weight of anadditional oxide substance that is a mixture 1 1 of oxides selected fromthe group consisting of oxides of Li, Na, K, Be, Ca, Ba, Al, B and Siand having a melting point below 1400 C., said oxide substance being inthe form of a coating on the silicide particles in the pores and at thesurface of the sintered body.

3. A sintered electric resistance body having a hightemperature portionsuitable for operating temperatures above 1300 C. and end portions fortemperatures below 1300 C., said body consisting essentially ofmolydenum silicide and containing in said end portions 0.1 to 20% byweight of additional oxide substance consisting of at least one oxide ofacidic character selected from the group consisting of oxides of Li, Na,K, Be, Ca, Ba, Al, B and Si, having a melting point below 1400 C., saidoxide substance being in the form of a coating on the silicide particlesof the sintered body.

4. A sintered electric resistance body having a hightemperature portionsuitable for operating temperatures above 1300 C. and end portions fortemperatures below 1300 C., said body consisting essentially ofmolybdenum silicide with minor inclusions of high-melting oxides andcontaining in said end portions 0.1 to 20% by weight of additional oxidesubstance that is a mixture of oxides selected from the group consistingof oxides of Li, Na, K, Be, Ca, Ba, Al, B and Si and having a meltingpoint below 1400 C. and forming a glaze upon the silicide particles ofthe body, said oxide substance having a linear coefficient of thermalexpansion between 5- l to 8- 10- rnm./ C.

5. In a sintered resistance body according to claim 1, said additionaloxide substance being uniformly distributed for constant area density ineach cross section of said end portions of the body.

6. In a sintered resistance body according to claim 1, said additionaloxide substance having a quantitative distribution in said end portionsthat possess a concentration gradient from said high-temperature portionalong said end portions so that the percentage of said oxide substanceincreases with decreasing operating temperature along said end portions.

7. In a sintered resistance body according to claim 1, said additionaloxide substance having a melting point between 1250 and 1350.

8. In a sintered resistance body according to claim 1, said additionaloxide substance having a melting point between 1250 and 1350 C., andhaving, when molten, a surface tension of approximately 400 (:20)dyn./cm.

9. In a sintered resistance body according to claim 1, said additionaloxide substance consisting at least in part of natural silicate.

10. In a sintered resistance body according to claim 1, said additionaloxide substance comprising a lithiumaluminum silicate of the compositionLi O-Al O -2SiO to Li O-Al O -SiO with 4 to 12% by weight of Li O.

11. A sintered electric resistance body having a hightemperature portionsuitable for operating temperatures above 1300 C. and end portions fortemperatures below 1300 C., said body consisting essentially ofmolybdenum silicide with minor inclusions of high-melting oxides andcontaining in said end portions 0.1 to by weight of additional oxidesubstance that is a mixture of oxides selected from the group consistingof oxides of Li, Na, K,

Be, Ca, Ba, Al, B and Si and having a melting point below 1400 C. andforming a glaze upon the silicide particles of the body, saidhigh-temperature portion and said end portions being fusion-bonded andforming jointly an integral molydenurn-silicide structure.

12. A resistance body according to claim 1, comprising respectivejunction discs located between said high-temperature portion and saidrespective end portions of said sintered molybdenum-silicide body, saiddiscs consisting also of sintered molybdenum silicide and beingsinterbonded with said portions.

13. In a resistance body according to claim 12, each of said junctiondiscs having an axial height of to 5 of its diameter.

14. In a resistance body according to claim 12, each of said junctiondiscs of sintered molybdenum silicide containing likewise additionaloxide substances as specified for said end portions of said body.

15. In a resistance body according to claim 12, each of said junctiondiscs having a larger diameter than at least one of the two portions ofthe body joined by said disc and enveloping the adjacent end of saidportion.

16. In a sintered molybdenum-silicide resistance body according to claim1, said end portions for temperatures below 1300 C. containing 0.01 to1.0% by weight of irongroup metal (Fe, Co, Ni).

17. In a sintered molymdenum-silicide resistance body according to claim1, said end portions for temperatures below 1300 C. containing 0.01 to1.0% by weight of nickel.

18. In a sintered molybdenum-silicide resistance body according to claim1, said end portions for temperatures below 1300" C. containing aresistance reducing addition of not more than 1.0% by weight and havinga specific electric resistance below 0.4 ohm-mm. /m. at normal roomtemperature.

19. In a sintered resistance body according to claim 1, saidhigh-temperature portion consisting substantially of a ternarymolybdenum-silicon-aluminum sinter alloy, and said end portions fortemperatures below 1300 C. consisting of oxide-containing molybdenumsilicide free of non-oxidically bonded aluminum.

20. In a sintered resistance body according to claim 1, said silicidebeing formed of a molybdenum-silicon-aluminum alloy which contains anoxide constituent having a melting point below 1400 C.

References Cited by the Examiner UNITED STATES PATENTS 1,745,173 l/193OLeonard 29-191 2,258,327 10/1941 Kramer 29191 2,992,959 7/1961Schrewelius 33 8330 3,001,871 9/1961 Thein-Chi et a1. 211 3,067,03212/1962 Reed et al. 75211 DAVID L. RECK, Primary Examiner.

HYLAND BIZOT, BENJAMIN HENKIN,

Examiners. R. O. DEAN, Assistant Examiner.

1. A SINTERED ELECTRIC RESISTANCE BODY HAVING A HIGHTEMPERATURE PORTIONSUITABLE FOR OPENING TEMPERATURES ABOVE 1300*C. AND AND PORTIONNS FORTEMPERATURES BELOW 1300*C., SAID BODY CONSISTING ESSENTIALLY OFMOLYBDENUM SILICIDE AND CONTAINING IN SAID ENTIRE END PORTIONS 0.1 TO20% BY WEIGHT OF ADDITIONAL OXIDE SUBSTANCE THAT IS A MIXTURE OF OXIDESSELECTED FROM THE GROUP CONSISTING OF OXIDES OF LI, NA, K, BE, CA, BA,AL, B AND SI AND HAVING A MELTING POINT BELOW 1400*C., SAID OXIDESUBSTANCE BEING IN THE FORM OF A COATING ON THE SILICIDE PARTICLES INTHE PORES AND AT THE SURFACE OF THE SINTERED BODY.